#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget w;
    auto lay = new QVBoxLayout(&w);
    auto pb = new QPushButton("Push me!");
    lay->addWidget(pb);
    auto tw = new QTreeWidget;
    lay->addWidget(tw);
    tw->setColumnCount(1);
    for (int i = 0; i < 3; ++i)
        tw->addTopLevelItem(new QTreeWidgetItem({ QString("Item %1").arg(i + 1) }));
    QObject::connect(pb, &QPushButton::clicked, tw, [&]() {
        for (int i = 0; i < 3; i++) {
            auto item = tw->topLevelItem(i);
            item->setForeground(0, QBrush(Qt::red));
            for (int j = 0; j < 5; ++j) {
                auto item2 = new QTreeWidgetItem({ QString("SubItem %1,%2").arg(i + 1).arg(j + 1) });
                item->addChild(item2);
            }
        }});
    auto lw = new QListWidget;
    lay->addWidget(lw);
    for (int i = 0; i < 10; ++i)
        lw->addItem(QString("Item %1").arg(i + 1));
    w.show();
    return a.exec();
}